Gemstones
Much like a diamond, the various combinations of a stone's cut, color, clarity, and carat weight, along with availability and accessibility, determine rarity. You will find a brief description of those features here. These explanations are based on the system for gemstone evaluation established by the Gemological Institute of America. For more information on GIA, please visit www.gia.org.
Color
Color is typically the most important value-setting factor for gemstones. All gems have a preferred color or a relatively small range of preferred colors. The more the color varies from this range—lighter or darker, more vivid or less—the less valuable the stone. Color is composed of three dimensions: hue, tone, and saturation.
•Hue refers to the impression of color usually noticed immediately, such as red, yellow or blue.
•Tone refers to the degree of lightness or darkness of an object.
•Saturation defines the degree of purity of a hue.
Cut
Cut refers to the shape or design of a stone, arrangement of facets, as well as the precision of the stone's proportions and finish. The cutting process reveals the beauty of a gem. Gemstones are cut into shapes we are familiar with such as oval, emerald, pear, round, and marquise. In addition, they can be carved or fashioned into almost any design imaginable. Proportions involve the balance and appeal of the basic design. Finish refers to the detail of the workmanship. A well-proportioned cut with a fine finish will show a stone's optical properties to its fullest potential. When all of the other factors are even (color, clarity, and carat weight), a better-cut gem will be more valuable.
Carat Weight
The size of a gemstone is measured, not by its dimensions, but by weight. One carat, the traditional unit of measurement for gemstones, is equal to approximately 0.2 grams. You may also hear the weight of a gemstone referred to in points. A point is equal to 1/100 of a carat; therefore a 75-point gemstone equals 0.75 carat. Two different gemstones may have the same dimensions but different weights. This is due to the specific gravity or density of the gem mineral. This difference can help gemologists identify a gemstone. Up to a certain point, the larger a stone is, the more rare it is and the higher the price it will command. For stones that commonly occur in larger sizes, the value may decrease if the gem reaches a size that makes it impractical for jewelry use.
Clarity
A gemstone's clarity grade is directly related to its rarity. Clarity refers to a gemstone’s relative freedom from clarity characteristics. Clarity characteristics include inclusions, which lie within the stone, or blemishes, which lie on the surface of a gem. The fewer clarity characteristics, the more rare the gemstone.
Each variety of gemstone has its own clarity standards. For example, Tanzanite is virtually inclusion-free, while Emerald almost always contains clarity characteristics. For this reason, Gemological Institute of America's grading system divides transparent colored gemstones into three clarity types. This allows gems to be more evenly evaluated as it takes into account the individual nature of each gemstone. Each type is further divided into five grading descriptions.
Clarity Types
Type I gems are often virtually free of clarity characteristics. The stones most commonly seen in the market include:
•Aquamarine
•Green Tourmaline
•Morganite
•Smoky Quartz
•Green Tourmaline
•Tanzanite
•Topaz – Blue
•Zircon – Blue
Type II gems normally contain clarity characteristics. The stones most commonly seen in the market include:
•Alexandrite
•Garnet
•Zircon – Green
•Amethyst
•Lolite
•Sapphire – all colors
•Zircon – Orange
•Ametrine
•Peridot
•Spinel - all colors
•Zircon - Red
•Citrine
•Ruby
•Tourmaline
•Zircon – Yellow
Type III gems almost always contain clarity characteristics. These include:
•Emerald
•Tourmaline – Watermelon
Clarity Grades
Type I: Usually eye-clean with no clarity characteristics visible to the unaided eye.
Eye clean – appears clean to the unaided eye.
Slightly Included - Minute inclusions difficult to see with the unaided eye.
Moderately Included - Minor inclusions somewhat easy to see with the unaided eye.
Heavily Included - Prominent inclusions that have a negative effect on appearance or durability.
Severely Included - Prominent inclusions that have a severe effect on appearance, durability, or both.
Type II: Typically shows some eye-visible clarity characteristics that do not detract from the overall beauty of the gem.
Eye clean – appears clean to the unaided eye.
Slightly Included -Minute inclusions difficult to see with the unaided eye.
Moderately Included - Noticeable inclusions apparent to the unaided eye.
Heavily Included - Prominent inclusions that have a negative effect on appearance or durability.
Severely Included - Prominent inclusions that have a severe effect on appearance, durability, or both.
Type III: Almost always contain eye-visible clarity characteristics.
Eye clean – appears clean to the unaided eye.
Slightly Included - Noticeable inclusions apparent to the unaided eye.
Moderately Included - Obvious inclusions very apparent to the unaided eye.
Heavily Included - Prominent inclusions that have a negative effect on appearance or durability.
Severely Included - Prominent inclusions that have a severe effect on appearance, durability, or both.
The various combinations of color, clarity, cut and carat weight are primary factors in determining the value of a gemstone.
Rarity
Some other factors that impact rarity are the abundance of a particular gem type and the geographic location where it is mined. Several gemstones are more abundant and are more easily accessible than others. Many other gems are found in remote locations, which makes mining virtually impossible. The economic and political environments of the region where the gemstones are located may also limit the accessibility of some gems.
With all of these considerations, it is truly an amazing and often challenging journey from the mine to the jewelry store.
Source: Gemological Institute of America